1. Field of the Invention
The present invention relates to the provision of support for very large and heavy loads while simultaneously permitting limited movement of such loads in response to applied forces, and particularly forces which are exerted in directions which vary. More specifically, the present invention is directed to bearings which are suitable for use in a marine environment and especially to replaceable bearings which are capable of supporting hatch covers. Accordingly, the general objects of the present inventions provide novel and improved methods and apparatus of such character.
2. Description of the Prior Art
While not limited thereto in its utility, the present invention is particularly well suited for use as a bearing on which a hatch cover of a sea-going vessel may be supported. Such hatch covers are often extremely large and, in the case of vessels designed to transport containerized cargos, loaded containers may be stored both in the ship's hold beneath the hatch cover and on top of the hatch cover. Hatch covers are customarily supported on pads which are provided on the ship's coaming. These pads contact the underside of support members provided on the peripheral portion of the hatch cover. The load being supported by the pads may be extremely large because of the size and weight of the hatch cover itself and, as noted above, because of the fact that containerized cargo will customarily be stacked on top of the hatch cover.
When a ship is in motion, as a result of wave action, the coaming will try to move relative to the hatch cover and, accordingly, forces will be applied to the hatch cover. In order to keep the forces applied to the cover within safe limits, i.e., to prevent hatch cover damage, a limited amount of relative motion between the cover and the coaming must be accommodated. Additionally, the ingress of salt water into the hold of the vessel in regions between the support pads must be prevented. The required weathertight seal is accomplished through the use of an elastomeric seal which is located about the underside of the perimeter of the hatch cover. The means by which the hatch cover is supported, in addition to permitting relative motion as discussed above, must also insure the integrity of the seal by maintaining the height of the gap to be sealed within design limits.
In the prior art the interface between a hatch cover and the cooperating support pads has been metal-on-metal. Relative sliding movement between the hatch cover and coaming in response to pitch and roll of the ship has resulted in excessive wear of the planar contact surface of the pads and particularly galling. Such surface wear, in turn, increases the possibility that the hatch cover will bind on the pad and stresses will thus be applied through the pad to the hatch cover causing structural damage. In addition, wear of the surfaces of the contact pads may result in failure of the hatch cover seal, i.e., the seal may be subjected to excessive compression and will thus fracture. Hatch cover seal failure is a major concern, particularly for vessels registered in the U.S., since the seals are tested while in a U.S. port by being subjected to a high pressure stream of water. The penalty for ineffective hatch cover seals, because they present the possibility of hold flooding with calamitous results, is loss of license. Repair or replacement of hatch cover support pads and a damaged hatch cover or hatch cover seal resulting from pad wear is, of course, a time consuming and expensive endeavor. Maintenance of hatch cover support pads is difficult since container vessels are typically in port only long enough to unload and reload and repairs and maintenance must be performed around the load/unload activities and within its time frame. Even when a vessel is dry docked for major repairs, the replacement or repair of the metallic hatch cover support pads is a time consuming, complex and expensive task.